The invention relates generally to imaging techniques and, more specifically, to the calibration of an X-ray detector.
Non-invasive imaging broadly encompasses techniques for generating images of the internal structures or regions of an object or person that are otherwise inaccessible for visual inspection. One of the best known uses of non-invasive imaging is in the medical arts where these techniques are used to generate images of organs and/or bones inside a patient which would otherwise not be visible. Examples of such non-invasive imaging modalities include X-ray radiography and other X-ray based imaging techniques, such as tomosynthesis.
For example a medical X-ray radiography system typically operates by projecting X-rays from an X-ray source through an imaging volume. A portion of the X-rays pass through, and are attenuated by, a portion of a patient, such as the chest or an arm or leg. The attenuated X-rays are detected by an array of detector elements that produce signals representing the attenuation of the incident X-rays. The signals are processed and reconstructed to form images of the imaged region.
For example, a digital detector may be comprised of an array of individual photodetectors disposed beneath a single, monolithic scintillator or individual scintillators. The scintillators typically generate optical light when impacted by X-rays. The photodetectors, in turn, detect the optical light and generate responsive electrical signals that can be read out and, based on the location of the photodetector on the panel, used to generate an image. In such systems, the degree of output signal generated by a photodetector in response to a given X-ray input is known as the gain of the photodetector.
Photodetectors, however, may vary in their ability to detect the optical light and/or in their ability to generate a responsive output signal. As a result, not all of the photodetectors of the detector array may generate an equivalent output signal in response to the same X-ray dose, i.e., individual photodetectors may have different, intrinsic gain functions. In systems where the X-ray source and the detector have a fixed geometry, i.e., the source and detector do not move relative to one another, calibration addresses these differences in gain by providing a correaction factor for each photodiode so that, in response to a known X-ray exposure, the gain differences between photodetectors can be compensated. For example, the array of photodetectors may be exposed to a uniform field of X-rays and corrections factors determined for each photodetector so that, after application of the respective correaction factor, each photodiode produces a uniform signal. In this manner, each photodetector can be corrected to generate a uniform signal in the presence of such a uniform field.
Calibration, however, is less effective in systems where the X-ray source and detector are not fixed relative to one another. In particular, in such systems, output differences between photodetectors may be the result not only of differences attributable to the photodetectors themselves but also the result of the relative geometry of the X-ray source and the detector during an exposure event. Therefore, a correaction factor derived at one source/detector geometry may not properly correct for photodetector output differences at other source/detector geometries. In such systems, it would be desirable to distinguish between the portion of the differences in photodetector outputs attributable to source/detector geometry and the portions attributable to the photodetectors themselves.